Pad eyes are normally attached by welding to at least one end of tubular goods. At least two pad eyes are used diametrically opposite, within several inches of the end of the tube. After they have served the intended function, the pad eyes are often cut away. The pad eyes are critical elements in that they may temporarily support a full length of well bore related pipe. Their strength is a matter of concern and they are usually certified capable of the load expected.
When in the vertical position the load is usually supported by a wire line, or bail, attached to the pad eye by a clevis and pin. The hole in the pad eye has to be somewhat larger than the pin to enable the pin to be inserted without much time loss. A pin in an oversize hole, when lifting along a vector transverse to the pin axis, engages the hole wall along a line. At the line the unit loads are often very high and exceed the yield strength of the pad eye material. Large loads deform the material until an area of engagement on the pin distributes the stress to an amount below the yield strength of the pad eye material. The pin is harder than the pad eye and deforms very little. A deformed pin could not be easily pulled back through the clevis eye.
A considerable amount of engineering goes into certification of the pad eye but little is known about the stress distribution in metal around a deformed hole. The deformed metal around a hole presents a complex mathematical shape relationship to the parent metal. Metal crack resistance is suspect in the presence of work hardening that attends deformation. The prospect of dropping a string of casing, tubing, or conductor pipe into a hole is a matter that few engineers can ignore. through the clevis and the pad eye hole. The holes are currently made larger than the pin to avoid loss of time in rigging up for lifting. Making the hole, in total, fit the pin is not practical but making it larger, for utility, gives rise to the problem presently addressed.